Method of forming stiffener ribs in a sheet metal panel

ABSTRACT

A method of forming intersecting stiffener ribs in a flat sheet metal panel so as to rigidify the panel and prevent it from flexing along mutually perpendicular axes by cutting one or more slit means at a predetermined location on the sheet, each slit means being defined by a pair of spaced side edges of equal length and originating from a common point, forming an elongated rib shaped depression in the set along a first axis which intersects one of the side edges so as to cause a predetermined length of the one side edge to be displaced out of the plane of the sheet and thereby cause the other of the side edges to move in the direction towards the first side edge, forming a second elongated rib shaped depression similar in shape to the first depression along a second axis which intersects the other side edge of the slit means so as to cause a length of the other side edge to be displaced out of the plane of the sheet and cause the adjoining side edge to move towards the other side edge and meet the latter at substantially the same elevation relative to the plane of the sheet, and finally welding the side edges of the slit means together.

United States Patent [191 Perger I [111 3,848,323 [4 1 Nov. 19, 1974 [75] Inventor: Julius M. Perger, Cleveland, Ohio [73] Assignee: General Motors Corporation,

Detroit, Mich.

22 Filed: Sept. 4, 19 73 21 Appl. No.: 394,151

[52] US. Cl 29/475, 29/457, 29/476, 113/] M, 113/116 F, 113/116 Y [51] Int. Cl. 823k 31/02 [58] Field of Search 29/475, 476, 482, 407, 29/457, 483; 113/1 N, l M, 116 C, 116 W, 116 F, 116 Y [56] References Cited UNITED STATES PATENTS 1,594,609 8/l926 Frease ll3/l l6 F 2,485,648 l0/l949 Norquist 29/476 X 2,641,825 6/1953 Adams 29/476 2,869,228 l/1959 Martin 29/475 X 3,388,446 6/1968 Phillips Primary Examiner-C. W. Lanham Assistant Examiner-Victor A. DiPalma Attorney, Agent, or Firm-Edward J. Biskup [5 7 ABSTRACT A method of forming intersecting stiffener ribs in a flat sheet metal panel so as to rigidify the panel and prevent it from-flexing along mutually perpendicular axes by cutting one or more slit means at a predetermined location on the sheet, each slit means being defined by a pair of spaced side edges of equal length and originating from a common point, forming an elongated rib shaped depression in the set along a first axis which intersects one of the side edges so as to cause a predetermined length of the one side edge to be displaced out of the plane of the sheet and thereby cause the other of the side edges to move in the direction towards the first side edge, forming a second elongated rib shaped depression similar in shape to the first depression along a second axis which intersects the other side edge of the slit means so as to cause a length of the other side edge to be displaced out of the plane of the sheet and cause the adjoining side edge to move towards the other side edge and meet the latter at substantially the same elevation relative to the plane of the sheet, and finally welding the side edges of the slit means together.

4 Claims. 13 Drawing Figures METHOD OF FORMING STIFFENER RIBS IN A SHEET METAL PANEL This invention in general concerns a method of rigidifying sheet metal panels and more particularly a method of forming stiffener ribs in a sheet metal blank so as to permit the latter to resist flexing.

It is common practice to place elongated depressions or ribs in flat sheet metal panels for purposes of rigidifying the panel and increasing the flexural strength thereof. In most cases, the ribbing is placed along parallel axes so that essentially a corrugated type structure is formed having stiffening strength along one axis but lacking comparable strength along an axis perpendicular to the rib. There has been a need to provide'ribs along two directions using a cold-forming operation, however, heretofore the methods utilized for forming the cross ribs have usually resulted in the metal failing due to the high tension involved or the sheet metal having wrinkles and distortions formed therein.

Accordingly, the objects of the present invention are to provide a method of cold-forming stiffener ribs in a sheet metal panel that extend along intersecting axes; to provide an inexpensive method of forming stiffening ribs of desired depth in a relatively thick sheet metal panel without causing wrinkles or distortions in' the panel; to provide a method of making separate corrugations in a sheet metal panel that meet at a common I juncture and are joined together by a welding operation; and to provide a method of forming a pair of integral stiffening sections in a sheet metal panel with the stiffener sections crossing each other and being formed therein by a cold-forming operation.

The above objects and others are realized in accordance with the invention by a method which fonns the intersecting stiffener sections in a flat sheet of deformable metal by first cutting or otherwise forming one or more slit means at a predetermined location on the sheet. Each slit means is defined by a pair of spaced side edges of equal length and originating from a common point, and more than one slit means can be combined in various shapes such as an X shape or Y shape depending upon the type of ribbing pattern that is desired. An elongated depression shaped as a rib or corrugation is then formed in the sheet along a first axis which intersects one of the side edges at an angle relative thereto so as to cause a predetermined length of the latter-mentioned side edge to be displaced out of the plane of the sheet and thereby cause the other of the side edges to move in the direction towards the first side edge. A second elongated depression similar in shape to the first depression is then formed in the sheet along a second axis which intersects the other side edge of the slit means at an angle relative thereto so as to then cause a length of the other side edge to be displaced out of the plane of the sheet and cause the adjoining side edge to move towards the other side edge and meet the latter at substantially the same elevation relative to the plane of the flat sheet. Afterwards, the side edges of the slit means are joined together by a welding operation or the like so as to cause the depressions along the first and second axes to be rigidly interconnected and serve as stiffener ribs for the sheet.

A more complete understanding of the invention will be derived from the following detailed description when taken with the drawings in which:

FIG. 1 is an isometric view showing a sheet metal panel incorporating stiffener ribs made according to the invention;

FIG. 2 is a plan view of a sheet metal blank having guide lines marked thereon and slit means formed therein preparatory to the formation of the stiffener ribs in the panel;

FIG. 3 is an elevational view of a portion of a press brake incorporating a V-die for making one of the stiffener ribs in the panel of FIG. 2;

FIG. 4 is a view taken on line 44 of FIG. 3 showing the shape of the V-die used in making one of the stiffener ribs;

FIG. 5 is a cross-sectional view taken on line 5-5 of FIG. 3;

FIG. 6 is a view similar to that of FIG. 5 but shows the press brake closed for forming one of the ribs in the sheet metal blank;

FIG. 7 is a view similar to that of FIG. 2 but shows the stiffener rib formed in the blank by the dies of FIGS. 3 and 4;

FIGS. 8 and 9 are plan views of the V-dies utilized in making the other two stiffener ribs in the sheet metal blank of FIG. 2;

FIG. 10 is an enlarged view of the X shaped slit means formed in the blank of FIG. 5;

FIG. 11 is a sheet metal blank ofa vehicle door that is to be formed with ribs in accordance with the present invention;

FIG. 12 is an enlarged view of one of the corners of the sheet metal blank of FIG. 11; and

FIG.'13 is a perspective view showing one corner of the sheet metal blank of FIG. 11 after the ribs are formed in the blank.

Referring to the drawings and more particularly FIG.

1 thereof a flat sheet metal panel 10 is shown having in tersecting V-shaped stiffener ribs formed therein utilizing the method according to this invention. As seen. the panel 10, in general, consists of a flat sheet metal por tion 11 in which a stiffener rib 12 extends along the longitudinal axis thereof. In addition, a pair of transverse stiffener ribs 14 and 16 are provided in the portion II one of which is normal to and crosses the rib 12 while the other is located at one end thereof. The ribs 12, 14 and 16 serve to rigidify the sheet metal portion 11 by providing stiffener sections in two directions.

As seen in FIG. 2, the sheet metal portion I l is shown as a blank prior to having the ribs l2, l4, and 16 formed therein and it will be noted that it is a flat rectangular sheet of steel that has a uniform thickness and is marked with guide lines for locating the positions of the stiffener ribs l2, l4, and 16. The guide lines are marked on one surface of the sheet metal blank and can be made using any form of scoring tool or marking instrument which will provide identifying lines that can be readily seen. As should be apparent, the guide lines serve to outline the area where the ribs are to be formed and in this regard it will be noted that parallel lines 18, 20, and 22 locate the position for rib 12. The spacing between the guides lines is determined by the type and size of die utilized for forming the rib into the panel. In this instance, as seen in FIG. 1 the rib 12 as well as ribs 14 and 16 are identical in the form and have a V-shape provided by the V-dies of FIGS. 4, 8 and 9 which are identical in cross-section. Accordingly, the

distance between the outer guide lines 18 and 22 will be substantially equal to the sum of the transverse dimensions of both faces on each V-die. Preferably, the ribbing will be of uniform cross-section and therefore, the distance between the outer guide lines for each rib to be formed will be the same. This is the case with the ribs l2, l4 and 16 in the panel of FIG. 1. Accordingly, the location of the stiffener rib 14 is marked by parallel guide lines 24, 26, and 28 while the location of the stiffener rib 16 is identified by the parallel guide lines 30, 32 and 34.

Afer the blank has been marked with the guide lines described above, the blank is formed with slit means as seen in FIG. 2. Thus, at the intersecting outlined area for ribs 12 and 14, and X-shaped opening is cut into the sheet metal blank while at the connecting outlined area for ribs 12 and 16, a Y shaped opening is cut into the sheet metal. Both the X- and Y-shaped openings are actually composed of two or more slit means and, in this regard, it will be noted that the X-shaped opening is made up of four slit means while the Y-shaped opening is formed with two slit means. In each case, the slit means is identical in configuration and is identified by reference numeral 36.

Morespecifically, the X-shaped opening is formed with two pairs of aligned slit means 36 with one pair lying aling an axis which extends between the points of intersection of guide lines 18 and 24 and guide lines 22 and 28 and the other pair lying along an axis that extends between the points of intersection of guide lines 22 and 24 and guide lines 18 and 20. Each slit means 36 originates at the point of intersection of the outer guide lines and is defined by a pair of diverging side edges 38 and 40 of equal length having an included angle that is bisected by the axis along which the slit means is formed.

The slit means 36 can be cut into the sheet metal blank with a saw or any other well known tool for cutting openings in sheet metal along thin lines. The size of the included angle between the side edges 38 and 40 of each slit means 36 will depend upon the type of stiffener rib that is being formed in the sheet metal blank, the object being that upon forming the intersecting ribs, the side edges 38 and 40 of each slit means will meeet at substantially the same elevation to permit the ribs to be joined together as will be described hereinafter.

The Y-shaped opening in the area of intersection of the guide lines for the stiffener ribs 12 and 16 is formed in a manner similar to that described above in connection with the X-shaped opening. In this case, however, the stiffener rib 16 is to be located at the end of the stiffener rib l2 and therefore rather than requiring two pairs of aligned slit means, only one pair is used arranged in a V-shape. Thus, one slit means 36 lies along an axis extending between the point of intersection between outer guide lines I18 and 30 and the point of intersection of lines and 32. The other slit means 36 lies along an axis extending between the point of intersection of lines 30 and 22 and the point of intersection of lines 20 and 32. In addition, a slot is formed in the blank starting at the point of intersection of lines 20 and 32 and along an axisin line with the guide line 20. The slot is defined by a pair of side edges 42 and 44 equally spaced from and parallel to the latter axis and constitutes the base leg of the Y-shaped opening.

The Y-shaped opening can be cut into the sheet metal blank in the same manner as the X-shaped opening. Thus, the slit means 36 will originate at the point of intersection of outer guide lines 18 and 30 in one case and at the point of intersection of guide lines 22 and 30 in the other case. As with the slit means used for making the X-shaped opening, each slit means will be defined by diverging side eges 38 and 40 of equal length. The slit means 36 of the Y-shaped opening, however, will connect with the side edges 42 and 44 to complete the letter Y configuration.

With the sheet metal blank provided with the X- and Yfshaped openings as described above. the blank is then ready to have the stiffener ribs l2, l4, and 16 formed therein. In forming the stiffener ribs into the blank, a press brake can be used of the type shown in FIG. 3. As is well known, a press brake is a long power press that will accommodate various shapes of dies. In this case, the upper movable portion 46 of the press brake rigidly supports a male V-die 48 which, as seen in FIGS. 4 and 5, has an outer configuration that will be used for shaping the stiffener rib 12. In cross-section the V-die 48 comprises inclined faces or surfaces 50 and 52 which respectively terminate at horizontally aligned flat surfaces 54 and 56 of the press brake portion 46. In plan view, as seen in FIG. 4, the V-die 48 is provided with V-shaped cut-out portions 58 and 60 which serve as clearance space for the side edge 40 ofthe rib l4 and also terminates with a V-Vshape to provide clearance for the side edges 40 of the Y-shaped opening. The lower stationary portion 62 of the press brake rigidly supports a female V-die 64 which as seen in cross section in FIG. 5, conforms to the shape of the male V-die member 48. The V-die has inclined surfaces or faces 66 and 68 which terminate at horizontally aligned fiat surfaces 70 and 72 so that upon closing the press, the inclined surfaces and the fiat surfaces of the male and female V-dies register with each other.

In forming the ribs l2, l4, and 16, the blank of FIG. 2 is first located in the press brake in a position where the guide line 20 will be vertically aligned with the center longitudinal axis or apex of the male V-die 48. In addition, the center of the X-shaped opening in the blank is positioned in vertical alignment with the point of intersection of the two cut-out portions 58 and 60 in the V-die 48. Once the blank is properly positioned, the upper portion 46 of the press is powered downwardly causing the sheet metal between the outer guide lines 18 and 22 to be bent out of the plane of the sheet to form the V-shaped stiffener rib 12 shown in FIGS. 6 and 7. During the forming operation, and as seen in FIG. 10, the side edge 40 of each slit means 36 moves inwardly towards the other side edge 38. This occurs because during the forming operation, the V-die 48 contacts the upper surface of the sheet metal blank and depresses a portion thereof into the female V-die 64. Inasmuch as the rib forming operation is not a stretching or drawing operation but a bending operation, the sheet on opposite sides of the guide line 20 is moved towards the guide line 20. This causes the side edge 40 of each slit means 36 incorporated with the X- and Y- shaped openings to move in the direction of the adjacent side edge 38. In addition, the side edges 42 and 44 of the base leg of the Y-shaped opening move into substantial engagement as seen in FIG. 7.

The press is then opened and the sheet metal blank is removed therefrom and turned for positioning at another location of the press brake that is provided with a male V-die 74 that in plan view has the configuration shown in FIG. 8. The V-die 74 is identical in size to V-die 48 and includes V--shaped cut-out portions 76 and 78 which will serve to provide clearance space for the pointed ends defined by side edges 38 of rib l2 during the forming of rib 14. The procedure for forming rib 14 is the same as that used for forming rib l2, and it will be noted that during the forming of rib 14 by V-die 74, the sheet metal on the opposite side of the outer guide lines 24 and 28 moves towards the guide line 26 and the side edge 40 of each slit means 36 of the X-shaped opening moves upwardly in the direction of the other side edge 38 and meets the latter at substantially the same elevation relative to the plane of the sheet metal blank.

Afterwards, the sheet metal blank is positioned at another location of the press brake that has the upper movable portion thereof provided with a V-die 80'seen ter of the V-die 74. In addition, the point of intersection of side edges 38 of the Y-shaped opening is located in vertical alignment with the apex of the V-shaped cut-out portion 82 in the V-die 74. Once again the press is closed causing the sheet metal on opposite sides of guide line 32 to move towards each other and the side edges 40 to move upwardly towards the side edges 38 and meet at the substantially same elevation so as to form the stiffener rib 16 of FIG. 1.

The above described operations will result in the sheet metal panel 10 having the stiffener ribs 12, 14, and 16 formed therein as shown in FIG. 1. In order to assure that the panel 10 will maintain a rigid form, the side edges 38 and 40 of each slit means are joined together. The preferred method of joining the side edges 38 and 40 is to weld them together resulting in a weld seam 84 being formed at each slit means 36 as well as at the base leg portion of the Y-shaped opening. After the welding operation has been completed, the panel is one integral member having the stiffener ribs 12, 14 and 16 formed therein so as to increase the flexural strength of the panel along axes which in this instance are mutually perpendicular to each other. As should be apparent, other methods of joining the side edges 38 and 40 together can be employed the object being to rigidly interconnect the sheet metal adjacentlthe side edge so that the ribs are integrally connected and serve to rigidify the panel along two directions. Also, although the stiffener ribs are formed in the panel along perpendicular axes the present method can be utilized for forming intersecting stiffener ribs at various angles while maintaining the outer side edges of the panel in the plane of the sheet metal portion 11.

FIG. 11 shows a sheet metal blank 86 that can be formed with stiffener ribs using the method described above and afterwards be used as a closure member or door. The sheet metal blank 86 has the outer form of a rectangle and also has a rectangular opening 88 formed therein which can be used for supporting a window. The blank is formed of a plain flat sheet of steel which, because of the size of the blank, is relatively flexible. In order to rigidify the blank, the latter can be provided with ribs in accordance with the invention. In this regard, the desired location for the ribs would be to place them parallel to and equally spaced from the straight edges 90, 92, 94 and 96 of the blank. Accordingly, the surface of the blank is scored with guide lines 98, 100, 102 for a rib parallel to edge 90, guide lines 104, 106, 108 for a rib parallel to edge 92, guide lines 110, I12, 114 for a rib parallel to edge 94, guide lines 116, 118, 120 for a rib parallel to edge 96. As explained hereinbefore, the spacing between the guide lines for each rib will be determined by the size and type of forming die that will be used for bending the metal and forming the depression therein.

-Slit means 122 are then formed at each corner of the blank. In this case, the type of ribs to be formed in the blank are the same and the spacing of each rib from the adjacent edge is equal, and therefore the slit means 122 in each corner will be identical. More specifically. each slit means 122 at each cornerwill lie along a central axis which will pass through the points of intersection I of the inner, center and outer guide lines. Thus, the slit means will be determined by the type of rib that is to beformed in the blank and the thickness of the sheet metal that is being used.

The slit means 122 is formed in each corner of the blank asexplained above and is cut therein by using a saw or any other method used for making cuts along thin lines in sheet metal. After cutting the slit means 122 in each corner of the blank, the latter is ready to be placed in a pres brake for forming the ribs. In this instance, the ribs to be formed in the blank are V-shaped and accordingly V-dies such as those used for forming the panel 10 can be used. The same V-die can be used for forming the rib along the edges 92 and 96 and, as should be apparent, will have a length that will conform to the distance bounded by the guide lines 104 and 108 and between the side edges 126 of the upper and lower slit means along edge 92. The ends of the V-die will be angled so as to provide clearance for the side edges 124 and thereby assure that only the metal between the side edges 126 of the upper and lower slit means 122 is subjected to the bending operation. Similarly, the same V-die will be used for forming the ribs along edges and 94 and the length of the V-die will correspond to the distance between the side edges 124 along edge 90. The latter V-die will also have the ends thereof angled so as to provide clearance for the side edges 126 and assure that only the metal between the side edges 124 along edge 90 and also edge 94 is subjected to the bending operation.

Assuming the ribs along edges 90 and 94 are to be formed first, the blank will be positioned in the press brake so that the guide line is vertically aligned with the longitudinal center of the V-die and the side edges 1240f the slit means are vertically aligned with the angled ends of the V-Vdie. The press is then closed causing the sheet metal on opposite sides of the guide line 100 to move towards each other with resultant movement of the side edge 126 of each upper slit means 122 towards the other side edge 124. The rib along edge 94 is formed in the same manner after the blank is properly aligned in the press using the guide line 112 and the side edges 124 for positioning relative to the V-die. The blank is then positioned in the press with the proper V-die therein for forming the ribs along the edges 92 and 96. Again the center-guide lines and the connecting side edges are used for obtaining correct alignment of the blank relative to the V-die. Upon closing the press, the sheet metal on opposite sides of the center guide line will move towards each other and the side edges 126 between the inner and outer guide lines will move upwardly to meet the side edge 124 at substantially the same elevation. As a result each corner of the blank will have intersecting ribs such as the ribs indicated by reference numerals 128 and 130 in FIG. 13. Afterwards, the side edge can be joined together by a welding operation which will provide a weld seam 132..

A blank successfully formed as described above was made from a hot rolled commercial quality pickled in oil sheet steel, 0.20 percent maximum carbon and measured l 1 gauge or 0.1 196 inches in thickness. The blank was 30 inches in width and was 50.88 inches long. The V-die used for forming the depressions or ribs in the blank had an included angle of 90 with each surface of the V having a transverse dimension of 1.25 inches. Accordingly, the spacing between the inner and outer guide lines along the edges 90, 92, 94 and 96 measured 2.50 inches and the outer guide line in each case was placed approximately 1.25 inches from the adjacent edge of the blank.

If desired a rib could be placed in the aforementioned door blank slightly below the window opening 88, parallel to the edges 99 and 94, and interconnecting the ribs 130 formed along edges 92 and 96. In such case, the blank would be marked with guide lines as explained above and the area of intersection of the guide lines along side edges 96 and 92 with the cross rib would each be provided with a Y-shaped opening as explained in connection with the panel 10.

Finally, it will be noted that a relatively small drilled hole can be made a part of each slit means 122 at the point of intersection of side edges 124 and 126. The purpose of the hole would be to relieve any stress concentrations which may occur at this point due to the bending operations described above.

Various changes and modifications can be made in this construction without departing from the spirit of the invention. Such changes and modifications are contemplated by the inventor and he does not wish to be limited except by the scope of the appended claims.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

E. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of placing a plurality of interconnected slit means in the interior of said sheet with each slit means including a pair of diverging side edges, forming an elongated depression in said sheet along a first axis intersecting one of said side edges at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, forming an elongated depression in said sheet along a second axis intersecting the other side edge at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause said one of said sideedges to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet. and joining said side edges together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet.

2. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of cutting a plurality of intercom nected slit means in the interior of said sheet with each slit means including a pair of spaced side edges of equal length, forming an elongated corrugation in said sheet along a first axis intersecting one of said side edges at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, forming an elongated corrugation in said sheet along a second axis intersecting the other side edge at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause said one of said side edges to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet, and welding said side edges of each slit means together so as to cause said corrugations along said first and second axes to be interconnected and serve as stiffener ribs for said sheet.

3. A method of forming intersecting stiffener sections in a flat sheet of deformable metal comprising the steps of cutting a plurality of interconnected slit means in said sheet so as to form an X-shaped opening with each slit means including a pair of spaced side edges ofequal length, bending the sheet so as to form an elongated depression therein along a first axis which passes through the center of said X-shaped opening and intersects one of said side edges of each of the slit means at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, bending the sheet so as to form an elongated depression therein along a second axis which passes through the center of said X- shaped opening and intersects the other side edge of each of the slit means at an angle relative thereto whereby said other side edge is displaced ut of the plane of said sheet to cause said one of said side edges of each slit means to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet, and permanently joining said side edges of each slit means together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet.

4. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of cutting a pair of interconnected slit means in said sheet in the form ofa V with each slit means including a pair of spaced side edges of equal length and originating from a common point, forming a slot in said sheet that connects with an outer edge and with said pair of slit means and together therewith forms a Y-shaped opening in the sheet, forming an elongated depression in said sheet along a first axis which passes through the slot and intersects one of said side edges of each of the slit means at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges and close said slot, forming an elongated depression in said sheet along a second axis which is perpendicular to the first axis and intersects the other side edge of each of the slit means at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet to cause said one of said side edges of each slit means to move in a direction towards said other side edge and meet the latter at substantially the same elvation relative to the plane of said flat sheet, and permanently connecting said side edges of each slit means of said slot together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet 313 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,848,323 Dated November 19, 1974 Inventofls) Julius M. Perger It is certified that error appears in the above-identified patent and that said. Letters Patent are hereby corrected as shown below:

r.- Ih the abstract, line 8, "set" should read --sheet-. I

Column 2, line 60, "guides" should read -guide-. Column 3, line 25, "aling" should read along---; line 43, "meeet" should read -meet-. Column 6, line 26, "beformed" should read --be formed--; line 33 "pres" should read --press--; line 59, "V-Vdie" should read -Vdie--. Claim 3, column 8,

line 48, "ut" should read --out--.

Signed and sealed this 4th day of March 1975.

( E EAL Attest:

C I- ARS HALL DANE-I U"""" Y-TF. L A1 L. {AUOJ Commissioner 0: Patents attesting O 'ficer and Trademarks 

1. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of placing a plurality of interconnected slit means in the interior of said sheet with each slit means including a pair of diverging side edges, forming an elongated depression in said sheet along a first axis intersecting one of said side edges at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, forming an elongated depression in said sheet along a second axis intersecting the other side edge at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause said one of said side edges to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet, and joining said side edges together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet.
 2. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of cutting a plurality of interconnected slit means in the interior of said sheet with each slit means including a pair of spaced side edges of equal length, forming an elongated corrugation in said sheet along a first axis intersecting one of said side edges at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, forming an elongated corrugation in said sheet along a second axis intersecting the other side edge at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet while maintaining the outer edges in the plane of the sheet so as to cause said one of said side edges to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet, and welding said side edges of each slit means together so as to cause said corrugations along said firSt and second axes to be interconnected and serve as stiffener ribs for said sheet.
 3. A method of forming intersecting stiffener sections in a flat sheet of deformable metal comprising the steps of cutting a plurality of interconnected slit means in said sheet so as to form an X-shaped opening with each slit means including a pair of spaced side edges of equal length, bending the sheet so as to form an elongated depression therein along a first axis which passes through the center of said X-shaped opening and intersects one of said side edges of each of the slit means at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges, bending the sheet so as to form an elongated depression therein along a second axis which passes through the center of said X-shaped opening and intersects the other side edge of each of the slit means at an angle relative thereto whereby said other side edge is displaced ut of the plane of said sheet to cause said one of said side edges of each slit means to move in a direction towards said other side edge and meet the latter at substantially the same elevation relative to the plane of said flat sheet, and permanently joining said side edges of each slit means together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet.
 4. A method of forming intersecting stiffener ribs in a flat sheet of deformable metal having outer edges comprising the steps of cutting a pair of interconnected slit means in said sheet in the form of a V with each slit means including a pair of spaced side edges of equal length and originating from a common point, forming a slot in said sheet that connects with an outer edge and with said pair of slit means and together therewith forms a Y-shaped opening in the sheet, forming an elongated depression in said sheet along a first axis which passes through the slot and intersects one of said side edges of each of the slit means at an angle relative thereto whereby said one of said side edges is displaced out of the plane of said sheet so as to cause the other of said side edges to move in the direction towards the said one of said side edges and close said slot, forming an elongated depression in said sheet along a second axis which is perpendicular to the first axis and intersects the other side edge of each of the slit means at an angle relative thereto whereby said other side edge is displaced out of the plane of said sheet to cause said one of said side edges of each slit means to move in a direction towards said other side edge and meet the latter at substantially the same elvation relative to the plane of said flat sheet, and permanently connecting said side edges of each slit means of said slot together so as to cause said depressions along said first and second axes to be interconnected and serve as stiffener ribs for said sheet. 